The only problem with this claim is obvious when you look at the enrollment by grade patterns over the past decade for Texas:

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Texas Enrollment by Grade 1997-2014
Click to enlarge image.

Notice the Promotion Rate line above in the chart, 5 lines from the bottom. Yes, Texas has made constant progress, but when only 79% of 9th graders are even making it to 12th grade, how do you have a graduation rate of 88%?

When the Cumulative Promotion Index, the last line, is only also 79%, how can you have a graduation rate of 88%. As multiple people are saying, this claim does not match with the data.